Chemical substances produced by microorganisms or synthetically that inhibit the growth of, or destroy, bacteria. Antibiotics are used at therapeutic levels to fight disease in humans and animals. Since the 1950s they have been used at subtherapeutic levels in animal feeds to enhance growth and prevent disease in livestock and poultry. Rules guiding the use of veterinary drugs and medicated animal feeds, including tolerance levels for drug residues in meats for human consumption, are promulgated by the centre for Veterinary Medicine of the Food and Drug Administration. The Food Safety and Inspection Service enforces the FDA rules through a sampling and testing programme that is part of its overall meat and poultry inspection program.